Engines may be configured with direct fuel injectors that inject fuel directly into a combustion cylinder (direct injection), and/or with port fuel injectors that inject fuel into a cylinder port (port fuel injection). Direct injection allows higher fuel efficiency and higher power output to be achieved in addition to better enabling the charge cooling effect of the injected fuel.
Direct injected engines, however, can generate more particulate matter (PM) emissions (or soot) due to diffuse flame propagation wherein fuel may not adequately mix with air prior to combustion. Since direct injection, by nature, is a relatively late fuel injection, there may be insufficient time for mixing of the injected fuel with air in the cylinder. In some operating conditions, the liquid fuel droplet may directly impinge on the combustion surfaces such as piston, head, and cylinder liner. Similarly, the injected fuel does not encounter turbulence when flowing through the valves. Consequently, there may be pockets of rich combustion that may generate soot locally, degrading exhaust emissions. The emissions may be further exacerbated during an engine cold start operation. In particular, until the combustion chamber is fully warmed up, soot is generated due to poor fuel evaporation caused by poor fuel injector spray characteristics at low fuel rail pressure and/or fuel impacting the cold metal surfaces of the combustion chamber.
Engine testing data indicates that PM emissions can be reduced by increasing engine temperature. Thus, an electric engine heater may be included in some engine systems. For example, as shown by Vigild et al. in US 20120291762, an intake heater is operated during DFSO conditions when engine fueling is deactivated. By heating air pumped to engine cylinders, the engine can be sufficiently heated to reduce soot emissions.
However, the inventors have identified potential issues with such an approach. As one example, due to the limited time available prior to an engine start, sufficient heating may not be possible. Likewise, due to the limited power availability on the vehicle, as well as the large engine mass, the available heating may not be sufficient. As such, the power required for sufficient engine heating may be higher, leading to poor fuel economy.
Some of the above issues may be addressed by taking advantage of various combinations of engine compression heating as well as individual cylinder compression stroke heating. One example method includes, while propelling a hybrid vehicle via motor torque, rotating an electrically-actuated intake compressor with an intake throttle closed and an EGR valve open until a piston temperature is higher than a threshold. Another example method includes, while propelling a hybrid vehicle via only motor torque, rotating an engine unfueled via the motor torque at lower than engine cranking speed while operating an exhaust heater coupled to an exhaust catalyst and while holding an EGR valve open and an intake throttle closed to recirculate heated aircharge through the engine. Still other combinations may be used in various heating modes. In this way, cylinder heating may be expedited before an engine is restarted.
As an example, while operating a hybrid vehicle in an electric mode, before an imminent engine start and in response to cylinder piston temperatures being not sufficiently hot, the engine may be slowly cranked, unfueled, via the hybrid vehicle's motor/generator to heat the engine cylinders. In one example, the slow cranking may be initiated at least 2-3 minutes before an engine start. The engine may be rotated slowly at lower than an engine cranking speed, such as at 10-30 rpm. During the slow engine rotation, each cylinder may be sequentially passed through a cylinder compression stroke where heat is transferred from the compressed air to the cylinder walls, head and piston. Even though the absolute amount of heat transferred to the engine may be low, the heat is transferred directly to a location where the heating enables a reduction is soot emissions when engine fueling is resumed. While rotating the engine, an intake throttle may be held closed while an EGR valve is held open so that the heated aircharge is pumped in a loop, further improving cylinder heat transfer. Optionally, one or more of an electric intake heater and an electric exhaust catalyst heater may be concurrently operated to further raise the temperature of the charge being circulated through the engine cylinders. In addition to raising cylinder temperature, the slow engine rotation enables a fuel rail pressure to be raised. Once the cylinder temperatures are sufficiently hot, such as when piston temperature is higher than a threshold, and if engine restart conditions exist, the engine may be rotated faster during cranking and engine fueling may be resumed.
In another example, while operating the hybrid vehicle in the electric mode, in response to a need for cylinder piston heating, an electric motor coupled to an electrically actuated compressor may be operated. During the compressor rotation, heat is generated during the compression of air. While the compressor is rotated, cylinder valve timing may be adjusted to increase valve overlap and improve blow-through of the compressed air through the engine cylinders. This enables heated from the heated aircharge to be transferred to the cylinders during the blow-through. While rotating the compressor, an intake throttle may be held closed while an EGR valve is held open so that the heated aircharge is pumped in a loop, further improving cylinder heat transfer. Optionally, one or more of an electric intake heater and an electric exhaust catalyst heater may be concurrently operated to further raise the temperature of the charge being circulated through the engine cylinders. In some examples, alongside the compressor rotation, a compressor recirculation valve may be opened so that compressor energy can also be used to warm a downstream charge air cooler. Further still, while rotating the compressor, the engine may also be slowly rotated, unfueled, so that the heated air can be uniformly distributed to all the engine cylinders. Once the cylinder piston temperatures are sufficiently hot, if engine restart conditions exist, compressor rotation may be disabled, the engine may be cranked, and engine fueling may be resumed.
In this way, by operating an electrically-actuated intake compressor prior to an engine restart, compressor rotation may be used to compress aircharge, thereby generating heat. By rotating the compressor with a compressor recirculation valve open, heat may be rejected from the compressed air at a downstream charge air cooler. By concurrently opening an EGR valve and closing an intake throttle, the heated aircharge can be looped across the engine. In addition, aircharge heating can be enhanced through the use of an intake or exhaust heater. By additionally or optionally slowly rotating the engine, unfueled, via motor torque, the pumped air can also be flowed through one or more cylinders, thereby warming the cylinders prior to an engine restart. In addition, compression stroke heating may be used to heat the cylinders. By pre-heating the engine, particulate emissions from the engine can be reduced, particularly during an engine cold-start. In addition, fuel pressure can be raised to an optimum value for the start, improving fuel injector spray characteristics during the restart. Overall, cold-start emissions can be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.